A turbocharger (for example, a turbocharger) is a device that increases the output of an engine by driving a turbine with the exhaust energy of the engine, driving a compressor coaxially linked to the axis of rotation thereof to compress air, and supplying high-density compressed air to the engine. A turbocharger in general includes a turbine housing that forms an outer shell of the turbine, a center housing (or a bearing housing) that rotatably supports the axis of rotation (or a shaft), and a compressor housing that forms an outer shell of the compressor. Also, the compressor housing has a scroll section which is formed in a spiral shape about the axis of rotation of an impeller of the compressor. The compressor compresses air taken in from an axial direction, supplies the compressed air from a circumferential end surface of the impeller to the scroll section, and discharges the compressed air out of the scroll section.
In the turbocharger (so-called turbocharger) that is mounted on a vehicle such as an automobile, an engine side is shifted to a low rotational state by an accelerator off signal in a case where the vehicle is rapidly decelerated from high-speed driving, and thus the compressed air in a scroll is less likely to be transferred to the engine. Also, the intake amount decreases due to the compressed air remaining in the scroll. However, since inertia causes the turbocharger to continue rotating, the compressor enters a surge state. In order to control the surge state, some turbochargers have a recirculation conduit that connects the scroll section of the compressor housing with an intake port of the compressor, and a recirculation valve that performs the opening and closing of the recirculation conduit. In this configuration, the recirculation valve is opened, the compressed air in the scroll section of the compressor housing is returned to the intake port of the compressor when the vehicle is rapidly decelerated (for example, refer to Patent Document 1 and Patent Document 2).
Patent Document 1 discloses the recirculation valve in which a valve seat is arranged in a casing of the turbocharger that is routable via a bypass passage, the bypass passage is closed by a seal lip when a valve unit is lowered onto the valve seat, and a discharge side of the turbocharger controlled by high pressure is separated from an intake side controlled by low pressure.
The Background Art section of Patent Document 2 discloses a configuration in which a bypass pipe connecting an intake pipe on a downstream side of the compressor of the turbocharger with an intake pipe on an upstream side, and a low pressure exhaust gas recirculation (EGR) system connecting an exhaust pipe on a downstream side of the turbine of the turbocharger with the intake pipe on the upstream side of the compressor of the turbocharger are arranged. The recirculation valve performing the opening and closing of the bypass pipe is arranged in the bypass pipe, and an EGR pipe connecting the exhaust pipe with the intake pipe, an EGR valve performing the opening and closing thereof, and an EGR cooler are arranged in the low pressure EGR system.